This invention relates to current sensing. In particular, it relates to sensing of currents in a highly inductive field where currents are high and small changes in the current are required to be sensed.
Different current sensors are known for detecting magnetic changes as an indication of a measurement of current. These sensors include a magnetic sensor in the nature of a Hall Effect element located in adjacency with magnetic laminations. The devices are usually clamped about one or more electrical conductors and the change of the magnetic field about the conductors is detected by the Hall Effect device as transmitted by the magnetic laminations. These changes are translated into a current measurement. The device is usually in the form of two semi-circular elements which open and close about a common hinge point so that it can be fitted about the conductor. A locking device or spring device secures the elements which are constituted by the laminations about the conductor. The Hall Effect device is located in a slot formed in the magnetic laminations in one of the semi-circular elements. Consequently, at least three breaks exist in the magnetic path of magnetic sensors in prior art devices. This limits the magnetic effectiveness of the device and hence the current sensing ability and sensitivity of the device.
In another known magnetic sensor, a magnetic core is formed of laminations of magnetic material strips which are located in a circular fashion about a conductor in a manner so that free ends abut each other. The laminations are of a rigid material in a single set and are permanently located about a conductor. A magnetic sensing winding is located over the laminations. The laminations are prestressed so that there is a closing force to maintain end portions in engagement with a minimum reluctance. Openings are provided in the strips to provide saturable sections to limit the magnetic flux induced in the core. Moreover, the device is responsive to the high saturation conditions and, hence, the magnetic material of the device is of low permeability.
In the prior art sensing devices known to the Applicants the magnetic strips or the lamination construction is of a rigid nature. Hence there is no ability to adjust the current sensing device to accommodate the best magnetic configuration relative to the conductor configuration about which it is placed. Moreover, the prior art devices do not include arrangements for sensitivity determinations of current in a highly inductive field where there are large current flows and where unwanted effects of the adjacent electromagnetic fields are overcome. There is accordingly a need to provide a current sensing device which overcomes these difficulties.
A need particularly arises for reading small fault currents, for instance, in electrical power generation systems. In such systems, there are extremely high currents generated and there is the possibility of small ground faults occurring, for instance, in the rotor, or the exciter associated with a generator. Such small currents need to be detected early and when very low in value so as to prevent unnecessary and costly downtime of an entire generator unit.
It is also desirable to be able to detect the existence of small rotor ground faults at times when the rotor is in either a state of shutdown, startup, or in operation. By the early detection of the low ground fault currents, and consequent preventive maintenance of the systems, expensive generator downtime is minimized. Particularly, by being able to detect ground faults during the operational state, wasteful downtime is avoided.
There is also a need to provide means for being able to determine whether the fault is in the generator or exciter sides of the electric power generator system. By being able to detect faults on the exciter side, it is easier and less expensive to shut down simply the exciter and not the generator rotor. Accordingly, there is a need to have a ground current detector for use in a fault system for a power generator which has the ability to determine on which side of the detector the fault is located.
In Patent Application Ser. No. 129,160, the technique is described by which a fault current is detected in a highly sensitive mode. By use of a synchronization signal and by dissipation of the effect of stray capacitance from the circuit, there is the ability to sense small levels of fault current. This system is employed in the ground fault current detection in rotor systems of the present invention.